Lower leasing costs, more efficient office space and the hope of projecting an image more appealing to millennials are leading hedge funds and large corporations to leave Manhattan's most-coveted business district for a rejuvenated Downtown and the trendy far West Side. The construction of gigantic new office buildings that allow tenants to use 20 percent less space is pressuring prices and driving firms like consultants BCG and law firm WilmerHale out of the tony Plaza District in Midtown Manhattan. In the latest potential blow to the area surrounding the elegant Plaza Hotel - the city's most coveted business address for decades - asset manager BlackRock Inc is poised to lease 850,000 square feet at a planned 58-story tower at the new Hudson Yards development on the far West Side.